The purpose of this study is to investigate variation in the fat-free body composition of children and youth as a function of maturation, sex and racial background so that a more accurate estimation of body fatness can be developed for such methods as densitometry, hydrometry, 40K spectrometry and anthropometry. Previous research using a cross-sectional design has established a lower density of the fat-free body in black and white children than for the adult. As a result, body fatness is overestimated in children when adult equations are used to estimate their body composition. The proposed investigation is designed to measure changes in the fat-free body composition in a longitudinal sample to provide more definitive estimates of the extent of chemical immaturity in children. Furthermore, additional populations of children with obesity or with a history of high physical activity will be studied for their variation in fat-free body composition and compared to the cross-sectional data base established during the past two years. Components of the fat-free body to be measured in all subjects include total body water using deuterium oxide dilution, bone mineral content of the cross-section of the radius and ulna by photon absorptiometry, body potassium by whole-body 40K spectrometry, body density by underwater weighing and functional residual lung volume, and anthropometry. The relationship between anthropometry and body density will also be studied by obtaining data from several laboratories throughout the country. In this way, it will be possible to establish interlaboratory standards for the anthropometric-body composition relationship in children. Thus, this research is designed to contribute to improved body composition estimates in children for both research and clinical applications. The widespread use of anthropometry for the estimation of body composition in children has not been properly established and is needed to improve methodological approaches leading to more accurate assessment of both overweight (obesity) and minimal weight conditions in the childhood population.